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FUELS

Definition of a chemical fuel: A chemical fuel is a substance, which produces a significant amount of heat energy and light energy when burnt in air or oxygen.

Classification of chemical fuels: Chemical fuels are classified as primary and secondary fuels. Fuels, which occur in nature, are called primary fuels. Fuels, which are derived from primary fuels, are called secondary fuels. Chemical fuels are further classified as solids, liquids and gases. A complete classification of fuels with examples is shown in the following Table.

Physical state Primary fuels Secondary fuels

SolidLiquidGas

Wood, coalPetroleum Natural Gas

Charcoal, coke Petrol, diesel, keroseneLPG

Importance of hydrocarbons as fuels: Fossil fuels contain mainly hydrocarbons. These hydrocarbons are important sources of energy in daily life. Hydrocarbons are used as energy sources in cooking, lighting, automobiles, production of electricity in thermal power plants etc. These hydrocarbon fuels meet 80% of the world’s energy demand. Thus hydrocarbons are important sources of energy.

Definition of calorific value of a fuel [Gross calorific value]: It is defined as the amount of heat liberated when unit quantity (1 kg or 1 m3 ) of a fuel is completely burnt in air or oxygen and the products of combustion are cooled to room temperature.

Definition of net calorific value: It is defined as the amount of heat released when unit quantity of a fuel is completely burnt in air or oxygen and the products of combustion are let off into the atmosphere.

S. I. unit of calorific value: For solids, calorific value is expressed in J kg-1 (Joules per kg). For gaseous fuels it is expressed in J m-3 (Joules / m3).

Determination of calorific value of a solid fuel using Bomb Calorimeter :

Principle: A known mass of the solid sample is burnt in excess oxygen. The surrounding water and the calorimeter absorbs the heat liberated. Thus the heat liberated by the fuel is equal to the heat absorbed by the water and the calorimeter.

Construction: The bomb calorimeter consists of a stainless steel vessel with an airtight lid. This vessel is called bomb. The bomb has an inlet valve for providing oxygen atmosphere

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inside the bomb and an electrical ignition coil for starting of combustion of fuel. The bomb is placed in an insulated copper calorimeter. The calorimeter has a mechanical stirrer for dissipation of heat and a thermometer for reading the temperature.

Working: A known mass of the solid fuel is placed in a crucible. The crucible is placed inside the bomb. The lid is closed tightly. The bomb is placed inside a copper calorimeter. A known mass of water is taken in the calorimeter. The bomb is filled with oxygen at a pressure of 25-30 atm. The temperature t1 in the thermometer is noted.

On passing an electric current through the ignition coil, the fuel gets ignited. The fuel burns liberating heat. The water is continuously stirred using the stirrer. The maximum temperature attained by the water, t2 , is noted.

Observation and calculations:

Gross calorific value = ( )

mtsww ∆+ 21 J kg-1

wherew = w1 + w2 = mass of water in the calorimeter, in kg + water equivalent of the calorimeter, in kgs = specific heat of water, in J kg-1 oC-1

∆t = t2-t1 = rise in temperature, in oCm = mass of the fuel, in kg

2

Thermometer

Lid

Wires for ignition

Sample

Oxygen

B

Stirrer

A

(Note: If the mass of fuel is given in grams, convert that into kg. For example, 0.2 g = 0.2 × 10-3 kg. If specific heat of water is given in J kg-1 oC-1, calorific value will be in J kg-1. If the specific heat is given in kJ kg-1 oC-1, then the calorific value will be in kJ kg-1.)

(Note: Specific heat of water is the amount of heat energy required to increase the temperature of one kg of water by one degree C.)

Problem 1. Calculate the calorific value of a sample of coal from the following data:Mass of Coal = 0.6 gMass of water + water equivalent of calorimeter = 2200 gSpecific heat of water = 4.187 kJ kg-1 oC-1

Rise in temperature = 6.52 oC

Solution: (Note: In solving the problem, follow the steps given below:

1. Write the given quantities and convert them into appropriate units.2. Write the equation.3. Substitute the values.4. Simplify using calculator if necessary.5. Write the answer.6. Write the units.)

Given: m = 0.6 g = 0.6 × 10-3 kgw1 + w2 = 2200 g = 2.2 kgs = 4.187 kJ kg-1 oC-1 = 4.187 × 103 J kg-1 oC-1

∆t = 6.52 oC

Gross calorific value = ( )

mtsww ∆+ 21 J kg-1

= 33

106.052.610187.42.2

−××××

= 1.001 × 108 J kg-1

Problem 2. A 0.85 g of coal sample (carbon 90 %, H2 5%, and ash 5% ) was subjected to combustion in a bomb calorimeter. Mass of water taken in the calorimeter was 2000 g and the water equivalent of calorimeter was 600 g. The rise in temperature was 3.5 oC. Calculate the gross and net calorific value of the sample. (Given, specific heat of water = 4.187 kJ kg-1 oC-1 and latent heat of steam = 2454 kJ kg-1 )

Solution: Given m = 0.85 g = 0.85 × 10-3 kg% of hydrogen = 5%w1 = 2000 g = 2 kgw2 = 600 g = 0.6 kg

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∆t = 3.5 oCs = 4.187 kJ kg-1 oC-1 = 4.187 × 103 J kg-1 oC-1

L = 2454 kJ kg-1 = 2454 × 103 J kg-1

a) Gross C.V. = ( )

mtsww ∆+ 21

C H2 C H 2 CH 2 CH 2 CH 2 C H2

Noval ac Resi n

OH OH O H O H O H OH OH

O H OH OH O H

+ HC HO +C H2

+ H 2O

P olym er isat ion

= ( )

3

3

1085.05.310187.46.02

−×××+

= 4.4825 × 107 J kg-1

b) Calculation of Net C.V.:

100 kg of coal contains 5 kg of H2 (given).

∴1 kg of coal contains 100

15 × = 0.05 kg of H2

W.k.t. 2 kg of H2 produces 18 kg of steam (from the equation

H2 + 1/2O2 H2O

∴ 0.05 kg of H2 produces 205.018 ×

= 0.45 kg of steam.

Heat released when 1 kg of steam condenses = 2454 × 103 J (given).

∴ Heat released when 0.45 kg of steam condenses = 1

45.0102454 3 ××

= 1.104 × 106 J

= 0.1104 × 107 J

= Q2

∴ Net C.V. = Q1 - Q2 = 4.4825 × 107 - 0.1104 × 107

= 4.3721 × 107 J kg-1

(Note: Latent heat of steam is the amount of heat energy liberated when one kg of steam is converted into one kg liquid water.)3. On burning 0.75g of a solid fuel in a bomb calorimeter the temperature of 2.5kg of water is increased from 240C to 280C the water equivalent of calorimeter and latent heat of steam are 0.485 Kg and 4.2X587 KJ/Kg, specific heat of water is 4.0KJ/Kg/0C

Solution: Given m = 0.75 g = 0.75 × 10-3 kgw1+W2 = (2.5+0.485) kg = 2.985 kg∆t = t2-t1=28-24=4 oCs = 4.2 kJ kg-1 oC-1 = 4.2 × 103 J kg-1 oC-1

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L = 2454 kJ kg-1 = 2454 × 103 J kg-1

a) Gross C.V. = ( )

mtsww ∆+ 21 =

KgJXKgJX

XXX /1066.6/1075.0

102.44985.2 73

3

=− C H2 C H 2 CH 2 CH 2 CH 2 C H2Noval ac Resi nOH OH O H O H O H OH OHO H OH OH O H

+ HC HO +C H2

+ H 2O

P olym er isat ion

Given 100g of coal contains 2.5g of H2

1 Kg of coal contains 2025.0100

5.2 Hofkg==

We know that 2 Kg of H2 produces 18 Kg of steam

0.025 Kg of H2 produces SteamKgX 225.0

218025.0 ==

Given, Heat released when 1Kg of steam releases = (4.2X 587) KJ/Kg=2465.4X103J/KgSo heat released when 0.225 Kg of steam released, Latent Heat

KgJXXX /1005547.01

255.0104.2465 73 ==

NCV = [GCV-Latent heat of steam] = [6.66 X 107 – 0.05547 X 107] J/ Kg = 6.60 X 107 J/Kg4. Calculate the gross and net calorific values by data given.

b) Mass of Coal = 0.7 g = 0.7 × 10-3 kgc) Mass of water = 2.2 kg d) Water equivalent of calorimeter = W2 = 0.25 Kg e) Raising temperature = 3.2 0C f) Specific heat of water = 4.187 kJ kg-1 oC-1 = 4.187 × 103 J kg-1 oC-1

g) Latent heat of steam = 580 calories / g [1 calorie = 4.18 joule]

m = 0.7 × 10-3 kgw1+W2 = 2.45 kg∆t = 3.2 oCs = 4.187 × 103 J kg-1 oC-1

L = 2454 kJ kg-1 = 2454 × 103 J kg-1

Solution:

h) Gross C.V. = ( )

mtsww ∆+ 21 = C H2 C H 2 CH 2 CH 2 CH 2 C H2Noval ac Resi nOH OH O H O H O H OH OH

O H OH OH O H

+ HC HO +C H2

+ H 2O

P olym er isat ion

KgJXX

XXX /10689.4107.0

10187.42.345.2 73

3

== −

Given latent heat of steam= 580 X 4.18 X 103 J /Kg = 0.2424 X 107 J/KgNCV=[GCV-Latent heat of steam] = [4.689 X 107 – 0.2424 X 107] = 4.446 X 107 J/Kg

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5. On burning 0.96g of a solid fuel in bomb calorimeter the temperature of 3500g of H2O increased by 2.70C water equivalent of colorimeter and latent heat of steam are 385 g and 587 cal/g respectively. If the fuel contains 5% H2 calculate its gross and net calorific value.Given: m=0.96g=0.96X10-3KgW1=3500g=3500X10-3KgW2=385g= 385X10-3Kg∆t=2.70CLatent heat =587 cal/g = 587X4.18X103 J/KgS= 4.187X103J/Kg/0C

Gross calorific value = ( ) KgJ

mtsww /21 ∆+

= 3333

1096.07.210187.4)10385103500(

−

−−

××××+ XX




15 × = 0.05 kg of H2

W.k.t. 2 kg of H2 produces 18 kg of steam (from the equation H2 + ½ O2 → H2O)

∴ 0.05 kg of H2 produces 205.018 ×

= 0.45 kg of steam.

Solve

6. Calculate the grass and net calorific value of coal sample from the fallowing data obtained from bomb calorimeter requirement Weight of coal sample taken = 0.085 KgWeight of water taken in the colorimeter= 1.4KgWater equivalent of colorimeter=0.47KgInitial temperature t1 = 250CFinal temperature= t2= 27.3 0CPercentage of H2 in coal sample = 5%Latent heat of steam = 587 X4.2KJ/Kg=587X4.2X103 J/KgSpecific heat of water = 4.187 KJ/Kg = 4.187 X 103 J/Kg

Gross calorific value = ( ) KgJ

mtsww

/21∆+

6

= 085.0

3.210187.487.1 3 ××× Solve




15 × = 0.05 kg of H2

W.k.t. 2 kg of H2 produces 18 kg of steam (from the equation H2 + ½ O2 → H2O)

∴ 0.05 kg of H2 produces 205.018 ×

= 0.45 kg of steam.

CRACKING Definition of cracking: Cracking is defined as the process of converting high molecular weight hydrocarbons into lower molecular weight hydrocarbons.

C14H30 C7H16 + C7H14

Department of Chemistry, Sambhram Institute of technologyFluidized bed catalytic cracking:

Principle: In fluidized bed catalytic cracking, the powder catalyst is kept agitated by gas streams (cracking fuel) so that the catalyst can be handled like a fluid system. This also results in a good contact between the catalyst surface and the reactant. Construction: A schematic diagram of fluidized bed catalytic cracking method is shown in the following figure.

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Optimum conditions: Catalyst used: y- type zeolite activated with a rare earth oxide(Al2O3+ SiO2)Temperature: 5500 C

Working: The feed stock vapors are passed into cracking chamber. The reactants undergo cracking in the presence of catalyst. The products are passed to a fractionating column.

Spent catalyst from the cracking chamber is continuously transported into the regeneration chamber through an air stream. The carbon deposited on catalyst particles is burnt off in regeneration Chamber. The regenerated catalyst is transported back into the cracking chamber together with feed stock.

REFORMATION

Definition of reformation of petrol: Conversion of straight chain hydrocarbons in petrol into branched chain, cyclic and aromatic hydrocarbons, resulting in upgradation of petrol is known as reformation.

Department of Chemistry, Sambhram Institute of technology

How reformation improves the quality of petrol?

The octane number for straight chain hydrocarbons is low. For branched chain, cyclic and aromatic hydrocarbons, the octane number is high. Thus reformation converts the low octane number petrol into high octane number petrol.

Reformation reaction conditions:

Reactant (feed stock) : Gasoline obtained by fractionation of petroleum + H2 Catalyst: Platinum supported on alumina (Pt / Al2O3 )Temperature: about 500 oCPressure: 15-50 atm.

Reformation reactions:

The main reformation reactions are:1) Isomerization, 2) Dehydrogenation and 3) Cyclization and dehydrogenation.

1) Isomerization:

Example: n – Heptane → 5 - Methyl hexane

8

H3C CH2 CH2 CH2 CH2 CH2 CH3

n-Heptane

H3C CH CH2 CH2 CH2

CH3

CH3

2-Methyl-hexane

2) Dehydrogenation:

Example: Cyclo hexane → Benzene + Hydrogen

C6H12 C6H6 + 3H2Cyclohexane Benzene

3) Cyclization and dehydrogenation

CH3 CH2 CH2 CH2 CH2 CH3 C6H12 + H2

C6H12 C6H6 + 3H2

n-Hexane Cyclohexane

Cyclohexane Benzene

CH3 CH2 CH2 CH2 CH2 CH2n-Heptane

CH3CH3

Methyl cyclohexane

Knocking in petrol engines: In petrol engines, the mixture of petrol and air is drawn in to the cylinder. The fuel-air mixture is compressed by the piston and is ignited by an electric spark. As the flame front travels in the combustion chamber, rapidly expanding combustion products compress the remaining unburnt fuel and raise its temperature. If the flame front travels rapidly at an optimum speed, the combustion of unburnt fuel takes place smoothly. On the other hand, if the flame front travels too slowly, the entire last portion of fuel mixture may get heated up beyond its ignition temperature and undergo instantaneous explosive combustion. This result in emission of a characteristic rattling sound called “knocking”.

Definition of knocking: Knocking may be defined as the production of a shock wave in an IC engine as a result of an explosive combustion of fuel-air mixture, leading to a rattling sound.

Mechanism of knocking in chemical terms:

9

A. Under normal conditions there is a slow oxidation of the fuel. It involves following steps:

1) Oxygen combines with hydrocarbon molecule (for example, ethane) forming peroxides.

H3C CH3 + O2 H3C O O CH3

Ethane Ethane peroxide

2) The peroxide molecules combine with other hydrocarbon molecules initiating a chain reaction.3) Slow combustion takes place due to the chain reaction. The overall reaction may be represented asC2H6 + 31/2O2 2CO2 + 3H2O

B. Under knocking conditions, the rate of combustion is very high. It involves following steps.

1. Oxygen combines with hydrocarbon molecule forming peroxides.

H3C CH3 + O2 H3C O O CH3

Ethane Ethane peroxide

2. The peroxides decompose readily to give a number of gaseous products. For example,

H3C O O CH3

Ethane peroxide

CH3 CHO + H2O

Acetaldehyde

CH3 CHO

Acetaldehyde

+ 11/2O2 HCHO + CO2 + H2O

Formaldehyde

Formaldehyde

HCHO + O2 H2O + CO2

2) Fast reaction leads to rapid increase of pressure. This results in knocking.

Definition of octane number: The octane number of a gasoline (petrol) is the percentage volume of iso-octane in a mixture of iso-octane and n-heptane, which has the same knocking property as the gasoline under test.

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CH3 C

CH3

CH3

CH2 CH

CH3

CH3

2,2,4 Tri Methyl Pentane

H3C CH2 CH2 CH2 CH2 CH2 CH3

n - Heptane

Antiknocking agent: Antiknocking agent is a chemical substance added to petrol to improve the Antiknocking property of the petrol. For example, tetra ethyl lead (TEL). Addition of TEL to petrol increases the octane number of petrol. TEL is normally used along with ethylene dibromide or ethylene dichloride. [Formula of tetra ethyl lead = (C2H5)4Pb ]

Unleaded petrol: Unleaded petrol is the petrol where tetra ethyl lead (TEL) is not used as an antiknocking agent.

Addition of TEL to petrol leads to release of PbBr2 and PbCl2 into the atmosphere through automobile exhaust. This results in air pollution. To prevent air pollution, unleaded petrol is preferred.

In unleaded petrol, methyl-t-butyl ether (MTBE) and ethyl-t-butyl ether (ETBE) are used as antiknocking agents.

Cetane number: The cetane number of a diesel fuel is the percentage by volume of n-cetane in a mixture of n-cetane and α-methyl naphthalene which has the same knocking characteristic as the diesel under test.

H3C (CH2)14 CH3

n-cetane

= C16H34 = Hexadecane C10H7 CH3 Methyl naphthalene


POWER ALCOHOL

Definition of power alcohol: When ethyl alcohol is used as an additive to motor fuels to act as a fuel for internal combustion engines, it is called power alcohol.

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The importances of power alcohol as fuel are: (Characteristics of alcohol-blended petrol are: )

1. The power output is good.2. Addition of alcohol reduces the emission of carbon monoxide and volatile organic

compounds in to the atmosphere.3. Alcohol improves the antiknocking property of the fuel.4. Petrol-alcohol mixture has the same lubrication as the pure petrol has.5. Since ethanol is produced from agricultural products, it can be a sustainable fuel.6. Ethanol is biodegradable.

Synthetic petrolBergius process:The low grade coal is finely powdered and made into thin paste with 50% of heavy oil and catalyst (Fe, Mn & Mo) in a mixer. It is subjected to hydrogenation by adding hydrogen, under a pressure of 200 atm at 450oC in a converter. The resulting crude oil vapour is directly fed into a fractionating chamber. When gasoline (light oil), kerosene (middle oil), Wax (heavy oil) fractions are recovered. The heavy oil fraction is recycled with fresh pulverized coal.

Fischer-Tropsch process:

Franz Fisher- Hans Tropsch showed that carbon monoxide and hydrogen in the presence of catalyst Fe or Co at 180-250oC form a mixture of aliphatic hydrocarbons ranging from methane to wax. The natures of the products formed however depend on the temperature, pressure and catalyst used.

Process:1. Production of water gas: water gas (CO + H2) is obtained by passing steam over red hot coke. C + H2O (g) CO + H2(Water gas)

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CatalystCoal Powder

Mixer

H2

Frac

tioni

ng c

olum

n

Gases

Gasoline

Kerosine

Heavy oil

Converter 450oC

Water gas freed from dust and organic sulfur compounds. The synthesis gas (CO + 2H2) is obtained by blending water gas with hydrogen.2. Hydrogenation of carbon monoxide: synthesis gas (CO + 2H2) is compressed to 5-10 atm pressure and passed into a catalyst reactor. The catalyst, which is a mixture of cobalt (100 parts), thoria (5 parts) and magnesia (8 parts), is initially heated to 250oC. As a result of hydrogenation, saturated and unsaturated hydrocarbons are produced

n CO + 2nH2 CnH2n + nH2OUnsaturated hydrocarbons

n CO + (2n + 1)H2 CnH2n + 2 + nH2OSaturated hydrocarbons

The petroleum (Mixture of hydrocarbons) so formed is next fractionated to yield gasoline, kerosene, heavy oil and paraffin wax.

Doping of Silicon: It is the method of addition of impurity atom to a pure semi conductor to get desired extrinsic (p or n) semiconductors.In the preparation of solar grade silicon. Impure silicon is converted into vapors this vapors is condensed on the silicon rod. The vapor of doping material is passed in to the chamber, doping can be accompanied by simultaneous depositing a dopant with the semiconductor material.Diffusion Technique: It is a process of diffusing the impurity atoms into a silicon walfer by heating the thin walfer just below its melting point in an atmosphere of impurity. Ion implantation:A thin silicon walfer is exposed to a high energy beam of impurity ions, impurities are penetrated into the silicon. The extent of penetration is controlled by the energy of the ion beam.FUELSQuestions

13

Frac

tiona

ting

colu

mn

Converterat 250oC

[CO + H2]

H2

Gasoline

Kerosine

Heavy oil

Paraffin wax

1. What is a chemical fuel? OR Define chemical fuel. 1 Mark

2. (Imp) Give complete classification of chemical fuels with examples. 3 Marks

3. Define calorific value of a fuel. 1 Mark

4. Define gross calorific value and net calorific value of a fuel. 2Marks

5. Give the SI unit for the calorific value. 1 Mark

6. (Imp) Describe how the calorific value of a solid fuel is determined using bomb calorimeter OR describe the determination of calorific value of a solid fuel. 5 Marks

7. Give an example for cracking reaction. 1 Mark

8. (Imp) Describe fluidized bed catalytic cracking. 5 Marks

9. (Imp) What is reformation? OR What is meant by reformation of petrol? 1 Mark

10. (Imp) How reformation improves the quality of petrol? 2 Marks

11. Give the reformation reaction conditions. 2 Marks

12. (Imp) Give any three reformation reactions. 2 Marks

13. What is knocking? OR What is knocking in petrol engines? 3 Marks

14. (Imp) Explain the probable mechanism of knocking in chemical terms. 4 Marks

15. (Imp) What is octane number? OR Define octane number. 1 Mark

16. Give the structures of iso-octane and of n-heptane. 2 Marks

17. What are antiknocking agents? Give one example. 1 Mark

18. What is unleaded petrol? Why is it important? 2 Marks

19. What is cetane number? 1 Mark

20. Give the structures of n-cetane and α-methyl naphthalene. 2 Marks

21. What is power alcohol? 1 Mark

22. What are the characteristics of alcohol-blended petrol? OR What are the advantages of alcohol-blended petrol? 5 Marks

23. Explain the synthesis of synthetic petrol by Bergius process 5 Marks

24. Explain the synthesis of synthetic petrol by Fischer --Tropsch process. 5 Marks

Solar EnergyPhotovoltaic Cells:

Photovoltaic cells are semiconductor device which convert solar energy into electrical energy. (Photovoltaic cell is based on the principle of photoelectric effect).

Working of Photovoltaic Cell:

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• A typical silicon photovoltaic cell is composed of a thin wafer consisting of an ultra thin layer of phosphorous doped (n-type)

silicon on top of boron doped (p-type) silicon.• Hence a p-n junction is formed between the two.• A metallic grid forms one of the electrical contacts of the diode and allows light to

fall on the semiconductor between the grid lines.• An antireflective layer between the grid lines increase the amount of light transmitted

to the semiconductor.• The cell’s other electrical contact is formed by a metallic layer on the back of the

solar cell.• When light radiation falls on the p-n junction diode, electron-hole pairs are generated

by the absorption of the radiation• The electrons are drifted to and collected at the n- type end and the holes are drifted

to and collected at the p-type end.• When these two ends are electrically connected through a conductor, there is a flow

of current between the two ends through the external circuit.• Thus photoelectric current is produced and available for use.

Importance of Photoelectric cell:

• Photovoltaic cell provides enormous amount of energy from sun which is unlimited, inexhaustible and renewable.

• Photovoltaic cells can serve for both off grid and on grid application.• Photovoltaic cell produces no pollution so it is environment friendly.• Photovoltaic cell energy conversion is highly modular. This is important with respect

to the development of electricity supply systems in many rural and remote areas, where grid extension is economically not feasible.

• It provides power for space craft and satellite.• Photovoltaic can be used as roof integrated systems, providing power and also

serving as optical shading elements for the space below and preventing overheating in the summer.

• Developments in the field of Photovoltaic cells will boost the semiconductor industry and storage battery industries.

Advantages:

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e-

P- Type layer

n-type layer

Antireflective layer

Metal grid

Sunlight

h+

e-

Metallic layer

• Fuel source is vast and essentially infinite.• No emissions, no combustion or radioactive residues for disposal. Does not

contribute to global change or pollution.• Low operating cost (no fuel).• No moving parts and so no wear and tear.• High reliability in modules.• Can be integrated into new or existing building structures.• High public acceptance and excellent record.

Disadvantages:

• Sun light is a diffuse, i.c., it is relatively low density energy.• High installation cost.• Poor reliability of auxiliary elements including storage.• Energy can be produced only during the day time.

Physical properties of silicon relative to photovoltaics:

Physical properties of silicon are as follow:• Silicon is a semiconductor with band gap of 1.2eV at 25oC.• At atmospheric pressure, silicon crystallizes to diamond cube like structure.• Silicon contracts when melted and expand when solidify.• High refractive index limits the optical applications of silicon.• The absorption and transmission properties in the 0.4-0.5 µm wavelength spectra are

important in the performance of photovoltaic cells.

Chemical properties relevant to photovoltaics:• Silicon is stable in the tetravalent and has a strong affinity for oxygen, forming stable

oxides and silicates.• Silicon and carbon form a strong Si-C bond and stable products.• Silicon forms hydrides, and monosilane (SiH4) is key chemical compound for the

production of Amphrous silicon and the purification of silicon to semiconductor grade.

• Silicon forms trichlorosilane and tetrachlorosilane with chlorine, which are the intermediates and the by-products of the purification process in metallurgical grade silicon to semiconductor grade.

Production of solar grade silicon:

Solar grade silicon has impurities in the ppb level is required for polysilicon used in semiconductor industry. It can be prepared by using two methods.

1. The Siemens process:

This method is based on the thermal decomposition of trichlorosilane at 1100oC on heated silicon rod placed inside a deposition chamber.Trichlorosilane is prepared by hydrochlorination of metallurgical grade silicon in a fluidized reactor.

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Si + 3HCl SiHCl3 + H2

This reaction occurs at 350oC normally without a catalyst. The trichlorosiline formed is double purified through fractional distillation.

High purity SiHCl3 is then vaporized diluted with hydrogen and introduced into the deposition reactors. The gas is decomposed onto the surface of heated silicon seed rods, electrically heated to about 1100oC, and growing large rods of hyper pure silicon.

The reactions involved are:

2SiHCl SiH2Cl2 +SiCl4

SiH2Cl2 Si + 2HCl

SiHCl + H2 Si + 3HCl The Union Carbide process:

This process involves the following steps:The hydrogenation of tetrachlorosilane through a bed of metallurgical silicon is carried out in a fluidized bed reactor

3SiCl4 + 2H2 + Si 4SiHCl3

The trichlorosilane is separated by distillation while the unreacted tetrachlorosilane is recycled back to the hydrogenation reactor.The purified trichlorosilane is passed through a fixed bed column filled with quaternary ammonium ion exchange resin acting as catalyst. Trichlorosilane gets converted into dichlorosilane.

2SiHCl3 SiH2Cl2 + SiCl4The products are separated by distillation, tetrachlorosilane is recycled to the hydrogen reactor and dichlorosilane is passed through a second fixed bed column filled with quaternary ammonium ion exchange resin. Dichlorosilane is converted into silane.

3SiH2Cl2 SiH4 + 2SiHCl3The above products are separated by distillation and trichlorosilane is recycled to the first bed column. Silane is further purified by distillation and then pyrolized to produce polysilicon onto heated silicon seed rods mounted in a metal bell-jar reactor

SiH4 2H2 + Si

Doping of Silicon by Diffusion Technique:In this technique, a region of a semi conductor material is incorporated with dopant atoms by the different impurity atom into the crystal of the material without actually melting it. By this technique the extent of impurity penetration can be controlled to a very small thickness of the material. For example, a n-type silicon can be obtained by heating a silicon walfer below its melting point in an atmosphere of n-type impurity such as Phosphorus. The

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impurity atoms condense on the surface of the walfer diffuse into the crystal. P-type silicon can be obtained by heating a silicon walfer below its melting point in an atmosphere of P-type impurity such as Boran. The extent of diffusion is regulating by temperature and concentration of the impurity atom.

Si + PH3 N-Dopped siliconDiffusion

Si + BH3 B2H6 P-Dopped silicon

Department of Chemistry, Sambhram Institute of technology__________________________________________________________________________

ELECTRODE POTENTIAL&CELLS

Electrochemistry: It is a branch of chemistry which deals with the study of transformation of chemical energy into electrical energy and vice versa”

Electrochemical cells: “An electrochemical cell is a device which converts chemical energy into electrical energy or electrical energy into chemical energy”

Types of electrochemical cells: 1. Galvanic cells-are devices which convert chemical energy into electrical energy. Example-Dry cell, Lead-acid cell, Ni-Cd cell etc.2. Electrolytic cells-are devices which convert electrical energy into chemical energy.Ex: Rechargeable cells and batteries(during recharging).

Galvanic cells: The Daniel cell is an example of a galvanic cell. It consists of a zinc rod dipped in a 1M ZnSO4.This forms one half cell. A copper rod dipped in a 1M CuSO4 constitutes the other half cell. These two electrodes are connected internally by salt bridge and externally connected through a voltmeter. The Daniel cell is based on the red-ox reaction.

Salt bridge

+-_

e-

CopperCathode

ZincAnode

CuSO4

Solution

ZnSO4

Solution

18

Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)At the zinc electrode zinc goes into the solution as Zn2+ ions liberating electrons ie at zinc electrode oxidation takes place

Zn Zn2+ + 2e-

At copper electrode reduction of Cu2+ ions takes place with the deposition of metallic copper on copper rod by gaining electrons, ie at copper electrode reduction takes placeCu2+ + 2e- Cu

By IUPAC convention the electrode at which oxidation takes place is anode and the electrode where the reduction occurs is cathode.

In Daniel cell, zinc electrode is anode and copper electrode is cathode

Classification of galvanic cells:

1. Primary galvanic cell-is a one which the cell reaction is not completely reversible ie irreversible. Example-Dry cell.2. Secondary galvanic cell-is one in which the cell reaction is reversible ie they are rechargeable. Example-Lead-acid battery, Ni-Cd battery etc.3. Concentration cells: “A concentration cell is an electrochemical cell in which electrode materials and electrolytes of two half cells are composed of same material but the concentration of two solutions are different”

Ex- Cu/Cu2+ (M1) || Cu2+ (M2)/Cu

SINGLE ELECTRODE POTENTIAL (E): It can be defined as “the potential developed at the interface between the metal and solution, when it is in contact with a solution of its own ions”

OR“Single electrode potential is a measure of tendency of a given half cell reaction to occur as reduction, when it is in equilibrium with the other half cell”

Standard reduction potential (E o ): “Standard electrode potential is the electrode potential when the electrode is in contact with a solution of unit concentration; at 298K.If the electrode involves a gas then the gas at one atmosphere pressure”

EMF of a cell: “Emf is the potential difference between the two electrodes of a galvanic cell which causes the flow of current from one electrode to other”Emf of any cell can be calculated by using the formulaEMF= Ecathode-Eanode

Measurement of electrode potential: The absolute electrode potential of single electrode can’t be determined. This because a single electrode of a cell works only when it is coupled with another single electrode and doesn’t work independency.

19

Therefore, the electrode potential of single electrode can be measured by using some reference electrode whose valve is arbitrarily taken as zero at all temperature. Commonly used reference electrode is the standard hydrogen electrode.In order to measured the electrode potential of given electrode, it is coupled with SHE through salt bridge. Then the valve of emf of cell, E cell is measured using electronic voltmeter.For example, to measure the potential of the zinc electrode is coupled with SHE through a salt bridge and voltmeter, as in diagram.

The cell can be represented as Zn/Zn2+|| HCl (H+=1M)|H2=1atm|pt

From the deflection in the voltmeter, the anode and cathode of the above cell can be identified. In the above cell, it was found that the given Zn electrode acts ad anode and SHE acts as cathode.

E mf of above cell was found to be 0.76V E cell=E cathode-E anode E cell=E OSHE - Ezn/zn2+ 0.76=0-Ezn/zn2+ Therefore E OSHE Ezn/zn2+=-0.76V

Hence the electrode potential of zinc is –veSimilarly to measure the electrode potential of copper. The copper is coupled with SHE, to get a complete cell arrangement through the salt bridge and voltmeter, as in figure.

The cell representation is, HCl (H+=1M)|H2=1atm|pt|| Cu2+/Cu

From the deflection in the voltmeter, the anode is SHE and cathode is copper electrode. Emf of the above cell was found to be 0.34V

Therefore E cell=E cathode-E anode

Salt bridge

-_

e-

ZincAnode

ZnSO4

Solution1M HCl

H2 Gas

SHE

0.76 V

Salt bridge

e-

SHE

ZnSO4

Solution1M HCl

0.34V

H2G

as

20

E cell=ECu2+/Cu-ESHE 0.34 =ECu2+/Cu-0 ECu2+/Cu= 0.34V

Hence the potential of copper electrode is +ve.

Origin of single electrode potential:

When a metal rod is in contact with a solution of its ions, two types of reactions are possible. The metal shows the tendency to go into the solution as metal ion by losing electrons

M Mn+ +ne-

Simultaneously, ions from the solution tend to deposited as metal atoms.

Mn+ +ne- M

These two opposite tendencies will result in an equilibrium represented by the equation.

Mn+ +ne- M

There are two possibilities for the position of equilibrium.i).Suppose at equilibrium the forward reaction has occurred to a greater extent than the backward reaction. As a result the electrode surface develops a layer of +ve charges, which attracts a layer of –vely charged ions at the interface developing an electrical double layer or Helmholtz double layer.ii).If the backward reaction has occurred to a greater extent than the forward reaction at equilibrium electrode develops a layer of –ve charges, which attracts a layer of +vely charged ions at the interface, again establishing an electrical double layer or Helmholtz double layer.Formation of Helmholtz double layer gives rise to a potential difference across the layer. This potential difference between the metal and the solution at the interface is the single electrode potential.The outside HDL, there is a thin diffused region of ions called Gouy-Chapman Layer (GCL)..

Nernst equation for single electrode potential:Nernst equation gives a relationship between single electrode potential and standard electrode potential through concentration of metal ions at a particular temperature.The decrease free energy (-ΔG) represents the maximum amount of work that can be obtained from a chemical cell reaction.

-ΔG=W max……………………………1W max= nFE…………………………….2

+ -+ -+ -+ -

- +- +- +- +- +

M M

Electrical double layer or Helmohltz double layer

21

Where, n= No of electrons, E=Electrode potential & F= Faraday constant.Under standard condition-ΔGo= nFEo……………………………..3

Where, -ΔGo=Decrease free energy under standard condition, Eo=Standard electrode potentialConsider a reversible electrode reaction

Mn+ +ne- M ……………………….4According to Vanthoff’s reaction isotherm

cKRTGG ln.0 −∆−=∆− …………..5

Where, Kc=Equilibrium constantsSubstitute the value of Kc, we have

[ ][ ]+−∆−=∆− nM

MRTGG ln.0 …………6

Substitute the value of -ΔG &-ΔGo[ ]

[ ]+−= nMMRTnFEnFE ln.0 ………….7

Divide the equ(7) by nF

[ ][ ]+−= nM

MnFRTEE ln.0 ……………8

For a solid metal concentration is unity, so that [M]=1

Then, we have

[ ]+−= nMnFRTEE 1ln.0 ……………..9

[ ]++= nMnFRTEE ln.0 ……………10

[ ]++= nMnF

RTEE log.303.20 ………..11

Substitute the value of R=8.314J/K/mol,T=298K,F=96500C/mol to the equ(11)

[ ]++= nMn

EE log.0591.00 ………12

Equation 12 is the Nernst equation for single electrode.

Nernst equation for emf of cells:

+=

AnodeatSpeciesCathodeatSpecies

nEE cellcell ..

..log.0591.00

Types electrodes:

1. Metal-Metal ion electrode: An electrode of this type consists of a metal dipped in a solution containing its ions. Ex- Zn/Zn2+, Cu/Cu2+ etc.

22

2. Metal-Metal salt ion electrode: These electrodes consist of a metal in contact with a sparingly soluble salt of the same metal dipped in a solution containing anion of the salt.Example-Calomel (Hg|Hg2Cl2|Cl- , Silver- Silver salt electrode (Ag| AgCl |Cl-)

3. Gas electrode : Gas electrode consists of a gas bubbling about an inert metal wire, immersed in solution containing ions to which the gas is reversible. The metal provides electrical contact and facilitates the establishment of equilibrium between the gas and its ions.Example-Hydrogen electrode (Pt|H2|H+), Chlorine electrode (Pt|Cl2|Cl-)

4. Oxidation-Reduction electrode: An oxidation-reduction electrode is a one in which the electrode potential arises from the presence of oxidized and reduced forms of the same substance in solution. The potential arises from the tendency of one form changes into the other more stable form. The potential developed is picked up by an inert electrode like platinum.Example-Pt|Fe2+, Fe3+ Pt|Ce3+, Ce4+

5. Ion selective electrode: In ion selective electrode, a membrane is in contact with a solution, with which it can exchange ions.Example- Glass electrode.

Reference electrode: “Reference electrodes are the electrodes with reference to those, the electrode potential of any electrode can be measured”

Calomel electrode: Calomel electrode is a metal-metal salt ion electrode. It consists of mercury, mercurous Chloride and a solution of KCl. Mercury is placed at the bottom of a glass tube. A paste of mercury and mercurous chloride Is placed above the mercury. The space above the paste is filled with a KCl solution of known concentration. A platinum wire is kept immersed into the mercury to obtain electrical contact. Calomel electrode canbe represented as, Hg| Hg2Cl2|Cl-.

The calomel electrode can acts as anode or cathode depending on the nature of the other electrode of the cell.The net cell reversible electrode reaction is,

Hg2Cl2(s) + 2e- 2Hg(l) + 2Cl-

Electrode potential, [ ]20 log.303.2 −−= ClnF

RTEE

23

Saturated KCl

Mercury

Calomel paste

Pt wire

Porous disc

[ ],log.303.20 −−= ClF

RTEE Where n=2

[ ]−−= ClEE log.0591.00 at 298K

Therefore electrode potential of calomel electrode is depending upon the concentration of KCl.For saturated KCl solution , the potential is found to be 0.242VFor 1M KCl solution, the potential is 0.281VFor 0.1M KCl solution , the potential is 0.334VApplications: 1. It is used as secondary reference electrode in the measurement of single electrode.

2. It is used as reference electrode in all potentiometer determinations.

Silver- Silver electrode:Silver-Silver chloride is also a metal-metal salt ion electrode. It consists of a small sheet of platinum is first coated with silver by the electrolysis of an argantocyanide solution. Silver is partially converted into AgCl by making as an anode in KCl solution and passing a current of low density for 30 minutes and it is dipped in a solution of KCl.Cell representation is as follows

Ag|AgCl|Cl-Net half cell reaction isAgCl + e- Ag + Cl-

Electrode potent [ ],log.303.20 −−= ClnF

RTEE

Where, n=1

[ ]−−= ClF

RTEE log.303.20

[ ]−−= ClEE log.0591.00 at 298K

Hence, the electrode potential is dependent on the concentration of chloride ions.For 1N solution, the electrode potential is 0.223V and for saturated solution is 0.199V at 298K.

Ion selective electrode: “Ion selective electrode is one which selectively responds to a specific ion in a mixture and the potential developed at the electrode is a function of the concentration of that ion in the solution”Example: Glass electrode

Glass electrode: A glass electrode is an ion selective electrode where potential depends upon the pH of the medium.The glass electrode consists of a glass bulb made up of special type of glass which has low melting point and relatively high electrical conductivity. The glass bulb is filled with a solution of constant pH (0.1MHCl) and insert with a Ag-AgCl electrode, which is the Internal reference electrode and also serves for the external electrical contact. The electrode

24

dipped in a solution containing H+ ions. The electrode representation is, Glass|0.1M HCl|Ag-AgCl.Advantages

1. This electrode can be used to determine PH in the range 0-9, with special type of glass even up to 12 can be calculated.2. It can be used even in the case of strong oxidizing agents.3. The equilibrium is reached quickly.4. It is simple to operate, hence extensively used in various laboratories.

Limitations1. The glass membrane though it is very thin, it offers high resistance. Therefore ordinary potentiometers cannot be used; hence it is necessary to use electronic potentiometers.2. This electrode cannot be used to determine the PH above 12. Determination of pH using glass electrode:

Principle: When a thin glass membrane is placed between two solutions of different pH values, a potential difference arises across the membrane. The potential difference varies as the pH of these solutions varies. In practice, pH of one of these solutions is kept constant and therefore the electrode potential depends on pH of the other solution ic experimental solution.

Procedure: glass electrode is immersed in the solution; the pH

H+ Ion Solution

Ag-AgCl electrode

0.1 M Hcl

Glass electrode

25

pH Meter

Calomelelectrode

Glasselectrode

Solution of unknown pH

is to be determined. It is combined with a reference electrode such as a calomel electrode through a salt bridge. The cell assembly is represented as,

Hg| Hg2Cl2|Cl-||Solution of unknown pH|glass|0.1M HCl|Ag+|AgClThe emf of the above cell, Ecell is measured using an electronic voltmeter with a pH meter.The emf of the cell is given by

E cell=E cathode-E anode …………….1 E cell=E glass- ESCE …………….2Since E SCE is knowing emf the cell,E glass can be evaluated.

The potential of glass electrode EG is given byEG=E1-E2 …………3 Where, E1&E2 are the electrode potential of outer & inner membraneEG=[Eo+0.0591 log [C2]]-[Eo+0.0591 log [C1] ] ……..4 Where,C1 & C2 are the concentration inner & outer acid solutions.

[ ][ ]1

2log.0591.0CCEG = ……………5

Or

[ ][ ]2

1log.0591.0CCEG −=

[ ] [ ]21 log.0591.0log.0591.0 CCEG +−= ……6Since the H+ concentration inside the glass bulb is a constant. The first term on RHS of the above equation becomes a constant.

[ ]2log.0591.0tan CtConsEG += ………….7

[ ]++= HtConsEG log.0591.0tan …………8 Since, C2= [H+] pHtConsEG .0591.0tan −= …………9 Where pH=-log [H+]

Substitute the value of EG to eqa (2)scecell EpHtConsE −−= .0591.0tan ……….10

0591.0

tan scecell EEtConspH −−= …….…11

Concentration cells:” A concentration cell is an electrochemical cell in which the electrode material and the solution in both the electrodes are composed of the same substances but only the concentration of the two solutions (electrolytes) is different”A typical example of Zinc concentration cell is shown below.

26

It consists of two zinc electrodes are immersed in two different concentration of ZnSO 4 solutions .These two electrodes are externally connected by metallic wire and internally by a salt bridge. The cell can be represented as,

Zn/Zn2+ (C1)|| Zn2+ (C2)/ ZnBy convention left hand electrode is the anode and right hand electrode is cathode.

At anode Zn Zn2+(C1) +2e

At cathode Zn2+(C2) + 2e Zn

Zn2+(C1) Zn2+(C2)Net cell reactionThe net cell reaction is merely the change in concentration as a result of current flow.

Emf of concentration cell:We know that electrode potential depends upon the concentration of the electrolyte. By convention, the potential of the cell is

Ecell=Ecathode-Eanode ………..1

Ecell= [ ] [ ]12 log.0591.0log.0591.0 MnMnEG +−= ………2

[ ][ ]1

2log0591.0MM

nEcell = at 298K ………3

From equation (3) following conclusions may be drawn.

1. When the two solutions are of the same concentrations, [ ][ ]1

2logMM

and no electricity

flows. Hence, Ecell=0 2. When M2/M1>1 ie M2>M1, log M2/M1 is positive & electrode potential is positive.3. Higher the ratio of M2/M1, higher is the value of cell potential.

Solution for Problems:

Salt bridge

+-_

e-

ZincCathode

ZincAnode

ZnSO4

SolutionZnSO

4

Solution

C1 C

2

27

1. Calculate the potential of Ag-Zn cell at 298 K, if the concentration of Ag+ and Zn +2 are 5.2x10-6M and 1.3x10-3M respectively. E0 of the cell at 298K is 1.5V given: T=298K; E0cell = 1.5V.

[Ag+] =5.2x10-6M[Zn+2] = 1.3x10-3M

w.k.t 2

20

][][log0591.0 +

+

+=ZnAg

nEE cellcell

3

26

103.1)102.5(log

20591.05.1 −

−

+=xxEcell

Ecell = 1.27 V

2. An electrochemical cell consists of iron electrode dipped in 0.1M FeSO 4 and silver electrode in 0.05M AgNO3 . Write the cell representation cell reaction and calculate the emf of the cell at 298K. (The standard reduction potentials of iron and silver are –0.44V and 0.8V respectively).

Given: T=298K; E0Fe = -0.44V; E0Ag= 0.8V[Fe+2] =0.1M[Ag+] = 0.05Mcell representation: Fe(s)| FeSO4(0.1M) || AgNO3(0.05M) | Ag(s)w.k.t. Ecell = Ecathode -Eanode

Ecell = [ ] [ ]200 0591005910 ++ −−+ FenEAgnE anodecathode log.log.

[ ]2

200 ][log0591.0 +

+

+−=FeAg

nEEE anodecathodelcel

[ ]2

1.0]05.0[log

20591.0)44.0(8.0 +−−=cellE

Ecell = 1.19 V.

3. Calculate the potential of Daniel cell at 250 C, given the electrode potentials of Cu and Zn are 0.34V and –0.76V respectively.

Given: E0Zn = -0.76V; E0Cu= 0.34Vw.k.t Ecell = Ecathode -Eanode

Ecell = 0.34-(-0.76)Ecell = 1.1 V.

4 Write the electrode reactions and Calculate the EMF of the given cell at 298K, Ag (s)AgNO3 (0.018M)| | AgNO3 (1.2M)Ag(s).

At anode Ag(s) Ag+ + e-

At cathode Ag+ + e- Ag(s)

.

w.k.t

=

1

205910CC

nE cell log

.at 298K

28

=

01802105910

..log.cellE (n=1)

Ecell = 0.1078 V.

5. Calculate the voltage of the cell Mg(s)Mg+2 (1M) Cd+2 (7x10-11M) Cd(s), where E0cell=1.97V.

Given:E0cell = 1.97V[Mg+2] =1M[Cd+2] = 7x10-11M

][][log0591.0 2

20

+

+

+=MgCd

nEE cellcell

11107log2

0591.097.1 −+= XEcell

Ecell =1.97-0.3001Ecell =1.6699 V.

6. Write the half cell and net cell reactions for the cell

Cd(s)Cd+2 (0.01M)Cu+2 (0.5M) Cu(s)The standard reduction potentials of Cd and Cu are –0.4V and 0.34V respectively. Calculate the emf of the cell.

Given: E0Cd = -0.4V; E0Cu= 0.34V; [Cd+2] =0.01M; [Cu+2] = 0.5M

At anode: Cd Cd2+ +2e-

At cathode Cu2+ + 2e- Cu

Net cell reaction Cd + Cu2+ Cd2+ + Cu

[ ]22

00 ][log0591.0 ++

+−=CdCu

nEEE anodecathodecell

01.05.0log

20591.0)4.0(34.0 +−−=cellE

Ecell =0.74+0.0502Ecell =0.7902 V.

7. Calculate the emf of Copper concentration cell at 250 C, where the copper ions ratio in the cell is 10.

Given: 101

2

2

2

==+

+

CC

CuCu

anode

cathode

][][

w.k.t

=

1

205910CC

nE cell log

. ; at 298 K

)log(. 102

05910=cellE

Ecell = 0.0296 V.

8. Calculate the electrode potential at a copper electrode dipped in a 0.1M solution of

29

Copper sulphate at 298K, assuming copper sulphate to be completely dissociated. The standard electrode potential of Cu+2 /Cu is 0.34V at 298KGiven: T=298K; E0Cu= 0.34V

[Cu+2] =0.1M

[ ]++ += 202 05910 CunEE CuCu log.

at 298 K

).log(.. 102

059103402 +=+Cu

CuE

029603402 .. −=+Cu

CuE

310502 .=+Cu

CuE V

Questions:

1. What is single electrode potential? Derive the Nernst equation for single electrode potential.

2. Discuss the origin of electrode potential.

3. What are concentration cells? Deduce the expression for the EMF of a copper concentration cell.

4. Explain the construction of Ag/AgCl electrode. Give the half cell reaction.

5. Write a note on Calomel electrode.

6. What is an ion selective electrode? Explain its principle and working.

7. Write a note on glass electrode.

8. Explain how glass electrode can be used in the determination of a PH of a solution.

9. Explain the determination of electrode potential by using standard hydrogen electrode.

Department of Chemistry, Dr.SMCE, Byranayakanahally, Bangalore rural


30

_______________________________________________________________BATTERY TECHNOLOGY

Galvanic cell: Galvanic cell is a device for converting chemical energy into electrical energy through a spontaneous redox reaction.

Battery: It is a device consisting of two or more galvanic cells connected in series or parallel or both.

Principle components of a battery are:

1. An anode where oxidation occurs.2. A cathode where reduction occurs.3. An electrolyte, which is ionically conducting.4. A separator to separate anode and cathode compartments.

Operation of a battery during discharging and charging :

Discharge: During discharge, oxidation takes place at the anode and reduction takes place at the cathode. The reaction is a spontaneous reaction. Chemical energy is converted into electrical energy.

Example: Lead-acid battery, discharging:

At anode Pb + SO42- PbSO4 + 2e-

At cathode PbO2 + 4H+ + SO42- + 2e- PbSO4 + 2H2O

At anode: electrons are released to the external circuit.At cathode: electrons from the external circuit are consumed.

Charging: During charging, reverse reactions take place. The reverse reactions are non-spontaneous reactions. The battery is connected to an external d.c. power supply. Electrical energy is converted in to chemical energy.

Example: The reverse of the above reactions occur during charging.

Characteristics of a battery:

1. Voltage: The voltage of a battery mainly depends upon the emf of the cells which constitute the battery system. The emf of the cell depends on the free energy changes in the overall cell reaction. As given by Nernst equation, 2.303RT

31

Ecell= Eocell- ------------- log Q nFWhere Ecell =Ecathode- Eanode, and Q is the reaction quotient for the cell reaction at any stage of the reaction, which is the ratio of the product of molar concentration of the reaction product molecules to that of reactants.As it is evident from the above equation, emf of the cell and the voltage available from the battery is dependent on standard electrode potential difference between the cathode and anode, temperature and the extant of the cell reaction.2. Current: Current is a measure of the rate at which the battery is discharging. Higher the rate of spontaneous reaction, higher is the current. Higher the surface area of the electrodes, higher is the rate of reaction. Current is measured in A.

3. Capacity : Capacity is a measure of the amount of electricity that may be obtained from the battery. It is expressed in Ah (ampere hours). It is proportional to the amount of charge in Coulombs that may be transported from anode to cathode through the external circuit. The charge (C) in Coulombs is given by the Faraday’s relation:

C = M

Fnw ××

where w is the weight of active material present at one of the electrodes, n = number of electrons involved in discharge reaction, F = 96500 C/mol, and M its molar mass.

4. Electricity storage density : It is the amount of electricity stored in the battery per unit weight of the battery. i.e. it is the capacity per unit weight. It can be expressed in Coulombs/kg or in Ah/kg. The weight includes the weight of all components of the battery (i.e. total weight of active material, electrolyte, terminals etc.)

5. Energy density : It is the ratio of energy available from the battery to its weight ( or volume). If a battery can be discharged at a current i and at an average voltage E for a period of time t, then the energy density is given by (i.E.t) / w, where w is the weight of the battery. It may be expressed in either Joules/kg or Wh/kg (watt hour per kg).

6. Power density : It is the power per unit weight (or volume) of the battery. If a battery can be discharged at a current i and at a voltage E, then the power density is given by (i.E)/w where w is the weight of the battery. It may be expressed in W/kg.

7. Energy efficiency : The energy efficiency of a rechargeable battery is given by

ingchduringconsumedEnergyingdischduringreleasedEnergyefficiencyEnergy

arg100arg% ×=

8. Cycle life : It is the number of discharge – charge cycles possible in a rechargeable battery before failure occurs. A good battery must have high cycle life.

9. Shelf life : The duration of storage of a battery, at the end of which the battery is able to give required performance is called shelf life. A good battery must have long shelf life.

32

Classification of batteries:

1. Primary batteries : These are non-rechargeable batteries, e.g. Zn-MnO2 dry cell.2. Secondary batteries : These are rechargeable batteries, e.g. Lead-acid battery3. Reserve batteries : In these batteries, one of the active components (e.g. electrolyte) of

the battery is separated from the rest of the components. It is assembled just before the use. e.g. Mg-water activated battery.

Zn-MnO2 cell (Dry cell, Leclanche cell): It is a primary (non-rechargeable) cell. Cathode cap

zinc chloride andammonium chloride pastepaste of MnO2 and graphite powder Graphite rod (cathode)

outer coverZn container (anode)

Construction: It consists of a cylindrical zinc container acting as anode and a graphite rod placed at the centre of the container acting as cathode. The zinc container is in contact with a paste of NH4Cl and ZnCl2 acing as the electrolyte. The graphite rod is surrounded by a paste of MnO2 and graphite.

2NH4Cl + 2 OH- 2NH3 + 2H2O +2 Cl-

Zn2+ + 2NH3 + 2Cl- Zn(NH3)2Cl2

Cell reactions:

At anode : Zn Zn2+ + 2e-

At cathode : 2MnO2 + H2O +2e- Mn2O3 + 2OH-

Overall reaction : Zn + 2MnO2 + H2O Mn2O3 + Zn2+ + OH-

Secondary reactions :

Applications: Used in flashlights, portable radios, toys and related electric articles.

Lead-acid battery: It is a secondary (rechargeable) battery.

33

CH2 CH2 CH2 CH2 CH2 CH2

Novalac Resin

OH OH OH OH OH OH OH

OH OH OH OH

+ HCHO +CH2 + H2O

Polymerisation

Construction: The anode and cathode are made up of lead grids. The anode grid is packed with spongy lead. The cathode grid is packed with PbO2. 5M sulphuric acid is used as electrolyte.

Cell reactions: At anode Pb + SO42- PbSO4 + 2e-

At cathode PbO2 + 4H+ + SO42- + 2e- PbSO4 + 2H2O

Over all reaction Pb + PbO2 + 2H2SO4 2PbSO4 + 2H2ODischarge

ChargeUses: 1) Used as a power source for starting, lighting and ignition in automobiles. 2) Used in emergency lighting and in uninterrupted power supplies (UPS).

Zn-air cell

Construction: In zinc-air cell, granulated powder of zinc mixed with the electrolyte (KOH) acts as anode material. Cathode is a carbon-catalyst mixture. The anode can and cathode can act as terminals. The anode material is separated from the cathode material by an electrolyte absorbent separator. KOH is used as the electrolyte.

.

34

H2SO

4

Lead grid filled with PbO

2 (cathode)

Lead grid filled with spongy lead (anode)

Cell reactions:

At anode : Zn + 2OH-

At cathode : 1/2 O2 + H2O +2e-

Over all reaction Zn + 1/2 O2

ZnO + H2O + 2e-

2OH-

ZnO

Uses: Used as a power source in hearing aids. Used in electronic pagers. Used in various medical devices

Nickel-metal hydride battery:

Construction: In these batteries, electrodes are made of porous nickel foil or nickel grid, into which the active material is packed. The active material for the anode is a mixture of a metal hydride (such as TiH2, VH2, or ZrH2 ) and a hydrogen storage alloy ( such as LaNi5 or TiNi ). The active material for cathode is nickel oxy hydroxide, NiO(OH). An aqueous solution of KOH acts as the electrolyte. Polypropylene is used as the separator.

Cell reactions:

At anode : MH + OH-

At cathode : NiO(OH) + H2O + e-

Over all reaction: MH + NiO(OH)

M + H2O + e-

Ni(OH)2 + OH-

M + Ni(OH)2

Uses: Used in cellular phones, camcorders and laptop computers.

Li-MnO2 cell:

35

Cell can

Cathode

Separator

Anode

Sealing washer separator

Cell capContact spring

Construction: The anode is made of lithium metal. The cathode is made of MnO2. A solution of lithium halide in organic solvent acts as the electrolyte. The anode and cathode are separated by a non-owen polypropylene separator.

Cell reactions:

At anode : Li

At cathode : MnO2 + Li+ + e-

Over all reaction: Li + MnO2

Li+ + e-

LiMnO2LiMnO2

Uses: 1) Used as memory back up.2) Used in watches, calculators, cameras etc.

FUEL CELLS

Definition of a fuel cell: Fuel cell can be defined as a galvanic cell in which the electrical energy is directly derived by the combustion of chemical fuels supplied continuously.

Differences between a battery and a fuel cell

Battery Fuel cell1. Store chemical energy. Do not store chemical energy.2. Reactants are with in the cell. Reactants are supplied continuously.

3. Products remain in the cell. Products are continuously removed from the cell.

36

Advantages of fuel cells:

1. Theoretically, the efficiency can be 100%. In practice, the efficiency is 50-80% which is high compared to conventional methods.

2. Harmful products are absent. Hence fuel cells are environment friendly.3. No need of charging.4. Silent operation.5. No moving parts. Hence wear and tear is eliminated.


Different types of fuel cells:

1. Alkaline fuel cells: Alkaline fuel cells use an aqueous solution of potassium hydroxide as electrolyte. The electrodes can be built from low-cost carbon and plastics. They are used in emergency and portable power generation.

2. Phosphoric acid fuel cells: These fuel cells use 98% H3PO4 as electrolyte. Platinum particles dispersed on carbon act as catalyst at both anode and cathode. They are used to provide light and heat in large buildings.

3. Molten carbonate fuel cells: Here, molten mixture of carbonates is used as electrolyte. The anode is porous Ni and cathode is porous NiO. These cells find use in chlor alkali and aluminium industries.

4. Solid oxide fuel cells: In solid oxide fuel cells, Y2O3 - stabilized ZrO2 is used as the electrolyte. The anode is Co-ZrO2 or Ni-ZrO2. The cathode is Sr-doped LaMnO3. These cells are suitable for electric vehicles.

5. Solid polymer electrolyte fuel cells: These cells use a polymer membrane as electrolyte. The electrodes are made of porous carbon impregnated with Pt.

Hydrogen-oxygen fuel cell:

Construction: It consists of two electrodes made of porous graphite impregnated with an electro catalyst such as Pt. The electrolyte is an aqueous solution of KOH. The hydrogen gas is continuously supplied at the anode and the oxygen gas is supplied at the cathode. Hydrogen and oxygen diffuse through the electrodes and react with the electrolyte in the presence of catalyst. The electrolyte is kept hot so that the water formed evaporates off.

37Wicks for maintaining water balance

Porous graphiteelectrode coated with platinum electrocatalyst

Polystyrene sulphonic acid ion exchange membrane in KOH

O2 + H

2OH2

O2

Anode

H2

ee

1.23 VCathode

Cell reactions:

At anode : H2 + 2OH-

At cathode : 1/2 O2 + H2O + 2e-

Over all reaction: H2 + 1/2 O2

2OH-

H2O

2H2O + 2e-

Uses: Used as a electric power source for space vehicles.

Methanol – Oxygen fuel cell

Construction: It consists of anode and cathode made of platinum. Sulphuric acid acts as the electrolyte. A membrane is inserted adjacent to the cathode on the electrolyte side to minimize the diffusion of methanol into the cathode. Methanol – H2SO4 mixture is circulated through the anode chamber. Pure oxygen is passed through the cathode chamber.

O2

CH3OH

Excess Oxygen and water

Cathode

Membrane

H2SO

4 (electrolyte)

Anode

CO2

CO2

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Wicks for maintaining water balance

Cell reactions:

At anode : CH3OH + H2O

At cathode : 11/2 O2 + 6H+ + 6e-

Over all reaction: CH3OH + 11/2 O2

3H2O

CO2 + 2H2O

CO2 + 6H+ + 6e-

Uses: It is used in large-scale power production.

Department of Chemistry, Sambhram Institute of technologyBATTERY TECHNOLOGY

Questions

1. What is a galvanic cell ? 1 Mark

2. What is a battery ? OR Define a battery. 1 Mark

3. Distinguish between a galvanic cell and a battery. 2 Marks

4. What are the main components of a battery ? 2 Marks

5. Describe the reactions occurring in a battery during discharging and charging with an example. 4 Marks

6. Describe the following characteristics of a battery: 1. Voltage, 2. Current, 3. Capacity. 4 Marks

7. Explain the following characteristics of a battery: 1. Electricity storage density, 2. Energy density, 3. Power density. 4 Marks

8. Describe the following battery characteristics: 1. Energy efficiency, 2. Cycle life, 3. Shelf life. 3 Marks

9. Give complete classification of batteries with examples. 3 Marks

10. Describe the construction and reactions of a Zn-MnO2 cell. OR Describe the construction and reactions of a dry cell. 4 Marks

11. Describe the construction and reactions of a lead-acid battery. 4 Marks

12. Describe the construction and reactions of zinc-air cell. 4 Marks

13. Describe the construction and reactions of a nickel-metal hydride battery. 4 Marks

14. Explain the construction and reactions of a Li-MnO2 cell. 4 Marks

15. What is a fuel cell ? OR Define a fuel cell. 1 Mark

16. Distinguish between a battery and a fuel cell. 3 Marks

17. What are the advantages of fuel cells? 5 Marks

18. Mention the electrolyte and electrodes in the following fuel cells. 1. Alkaline fuel cell, 2. Phosphoric acid fuel cell, 3. Molten carbonate fuel cell, 4. Solid oxide fuel cell, 5. Solid polymer electrolyte fuel cell. 5 Marks

19. Describe the construction and reactions of a H2 – O2 fuel cell. 5 Marks

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20. Explain the construction and reactions of a methanol- oxygen fuel cell. OR Describe the construction and reactions of a MeOH – O2 fuel cell. 5 Marks

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Department of Chemistry, Sambhram Institute of technology__________________________________________________________________________

CORROSIONDefinition of corrosion: Corrosion is defined as the destruction of metals or alloys by the surrounding environment through chemical or electrochemical reactions.

Example: When iron is exposed to air in the presence of moisture, hydrated ferric oxide (rust) is formed.

Fe2+ + 2OH- Fe(OH)2

2Fe(OH)2 + 11/2 O2 + H2O Fe2O3.3H2O rust(brown colour)

Electrochemical theory of corrosion: According to electrochemical theory, when a metal such as iron is exposed to corrosive environment, following changes occur.A large number of tiny galvanic cells with anodic and cathodic regions are formed.

1. Oxidation of metal takes place at the anodic region. e.g. Fe → Fe2+ + 2 e-

The Fe2+ ions dissolve, so corrosion takes place at the anodic region.2. The electrons travel through the metal from the anodic region to cathodic region.3. Reduction of O2 or H+ takes place at the cathodic region.

O2 + 2H2O + ne- 4OH-

The metal is unaffected at the cathodic region.

4. Fe2+ and OH- ions travel through the aqueous medium and form corrosion product.

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OH- OH-Fe2+ Fe2+

Iron Metal

O2

H2O

Electrons

Cathodic regionAnodic region

Fe2+ + 2OH- Fe(OH)2

5 The corrosion product may undergo further oxidation to form rust.2Fe(OH)2 + 11/2 O2 + H2O Fe2O3.3H2O

Reactions:

Reaction at anodic region: As mentioned above, oxidation of metal takes place at anode.

Fe2+ + 2e- Fe

Reactions at cathodic region: At cathode, the reaction is either a) liberation of hydrogen or b) absorption of oxygen.a) Liberation of hydrogen ( in the absence of oxygen)

In acidic medium the reaction is

2H+ + 2e- H2

In neutral or alkaline medium, the reaction is2H2O + 2e- 2OH- + H2b) Absorption of oxygen ( in the presence of oxygen)In acidic medium, the reaction is 4H+ + O2 + 4e- 2H2O

In neutral or alkaline medium, the reaction is O2 + 2H2O + ne- 4OH-

Overall reaction:Fe2+ + 2OH- Fe(OH)2

2Fe(OH)2 + 11/2 O2 + H2O Fe2O3.3H2O

Different types of corrosion:

1. Differential metal corrosion :

Anode Cathode Anode CathodeZn metal Fe metal Fe metal Cu metal

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This type of corrosion occurs when two dissimilar metals are in contact with each other and are exposed to a corrosive environment. The two metals differ in their electrode potentials. The metal with lower electrode potential acts as anode and the other metal with higher electrode potential acts as cathode.

The anodic metal undergoes oxidation and gets corroded. A reduction reaction occurs at the cathodic metal. The cathodic metal does not undergo corrosion.

The reactions may be represented as follows:

Cell reactions:

At anode : M

At cathode : O2 + 2H2O + 4e- 4OH- (Reduction of oxygen)

Mn+ + ne- (Oxidation of metal M)

Higher the potential difference between the anodic and cathodic metals, higher is the rate of corrosion.

Example: When iron is in contact with zinc, iron acts as cathode and zinc acts as anode because zinc has lower electrode potential compared to iron. Therefore zinc undergoes corrosion. On the other hand, when iron is in contact with tin, iron acts as anode because iron has lower electrode potential compared to tin. Hence iron undergoes corrosion.

Other examples: 1. Steel screws in copper sheet.2. Steel screws with copper washer.3. Bolt and nut made of different metals.

2. Differential aeration corrosion :

This type of corrosion occurs when two different parts of the same metal are exposed to different oxygen concentrations. (e.g. An iron rod partially dipped in water.) The part of the metal which is exposed to less oxygen concentration acts as anode. The part which is

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Less O2, (Anode)

Water

More O2,

(Cathode)

Iron

exposed to more oxygen concentration acts as cathode. The anodic region undergoes corrosion and the cathodic region is unaffected.


Cell reactions:

At anode : M



Other examples:

1. Part of the nail inside the wall undergoes corrosion. 2. When a dirt particle sits on a metal bar, the part under the dirt undergoes corrosion.3. Partially filled iron tank undergoes corrosion inside water.

3. Pitting corrosion: This is an example of differential aeration corrosion.

When a small dust particle gets deposited on a steel surface, the region below the dust particle is exposed to less oxygen compared to the remaining part. As a result, the region below the dust particle acts as anode undergoes corrosion and forms a pit. The remaining region of the metal acts as cathode and is unaffected.


Cell reactions:

At anode : M



Formation of a small anodic area and a large cathodic area results in intense corrosion below the dust particle.

4. Water line corrosion : This is an example of differential aeration corrosion.

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Rust

Water

More oxygen, (Cathode)

Less Oxygen (Anode)

When a steel tank is partially filled with

water for a long time, the inner portion of the tank below the water line is exposed only to dissolve oxygen, where as, the portion above the water line is exposed to more oxygen. Thus the portion below the water line acts as anode and undergoes corrosion. The upper portion acts as cathode and is unaffected.

A distinct brown line is formed just below the water line due to the deposition of rust.


Cell reactions:

At anode : M



Other example: Ships which remain partially immersed in sea water for a long time undergo water line corrosion.

5. Stress corrosion :

Stress corrosion occurs when stressed region of metals are exposed to corrosive environments. The stressed region acts as anode and undergoes corrosion. The unstressed region acts as cathode and is unaffected.

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Cathode

Boiler

Soft water = Very dilute NaOH

Anode

Stress corrosion is highly specific in nature. For corrosion to occur, the presence of both stress and a specific corrosive environment are necessary. For example, stressed regions of mild steel undergo stress corrosion in the presence of NaOH solution.

Caustic embrittlement in boilers is an example of stress corrosion. Here alkaline (NaOH) water enters the hairline cracks on the boiler wall (mild steel). The water evaporates off leaving behind a concentrated solution of NaOH in the crack. When the concentration of NaOH increases to 10%, an electrochemical cell is set up with the stressed region acting as anode. The iron surrounded by dilute NaOH acts as cathode. The anodic region undergoes corrosion leading to crack propagation which may result in boiler failure.

Factors affecting the rate of corrosion:

1. Nature of the metal: Metals with lower electrode potentials are more reactive and are more susceptible to corrosion. For example, elements such as Mg and Zn, which have low electrode potentials, are highly susceptible to corrosion. Noble metal such as gold and platinum, which have higher electrode potentials, are less susceptible to corrosion.

Exceptions: Metals and alloys which show passivity are exceptions for this general trend. Such metals form a protective coating on the surface which prevents corrosion.

2. Nature of corrosion product: If the corrosion product is insoluble, stable and non-porous, then it acts as a protective film which prevents further corrosion. The film acts as a barrier between the fresh metal surface and the corrosive environment. On the other hand, if the corrosion product is soluble, unstable and porous, then the corrosion process continues even after the formation of corrosion product.

Example: Aluminium, titanium and chromium form a protective film of metal oxide on the surface. Stainless steel forms a protective film of Cr2O3 on the surface. But in the case of Zn and Fe, the corrosion products formed do not have protective value.

3. Difference in potential between anodic and cathodic regions: Larger the potential difference between the anodic and cathodic regions, higher is the rate of corrosion. For example, the potential difference between iron and copper is 0.78 V, and between iron and tin is 0.3 V. Therefore, corrosion is faster when iron is in contact with copper.

The use of dissimilar metals should be avoided wherever possible. Otherwise, the anodic metal gets corroded.

4. Anodic and cathodic areas: Smaller the anodic area and larger the cathodic area, more intense and faster is the corrosion. For example, a broken coating of tin on iron surface results in intense corrosion at the broken region. Iron is anodic to tin. Exposed region of iron acts as anode with small area. Tin acts as cathode which has large area.

5. Anodic and cathodic polarizations: The polarization at anode and cathode decreases the rate of corrosion.

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Anodic polarization occurs due to the accumulation of metal ions in the vicinity of anodic region. This retards the formation of new metal ions by oxidation. Thus the corrosion process is retarded.

Cathodic polarization occurs due to the accumulation of hydroxyl ions in the vicinity of cathodic region. This accumulation retards the movement of cathodic reactant (oxygen) towards the cathodic surface. Hence, the rate of cathodic reaction decreases. A retarded cathodic reaction results in retardation of anodic reaction also. Thus the corrosion process is slowed down.

6. Hydrogen over voltage: When the cathodic reaction is liberation of hydrogen, a high hydrogen over voltage retards the cathodic reaction. A retarded cathodic reaction retards the anodic reaction also. Thus the corrosion process is slowed down.

(Note: Theoretically, certain potential difference between anode and cathode is required for the hydrogen evolution to occur at cathode. But in practice, the potential difference required is more than the theoretical value. This excess potential difference is called hydrogen over voltage.)

7. p H of the medium : In general, lower the pH of corrosion medium, higher is the corrosion rate. (Exception: Metals like Al, Zn etc. undergo fast corrosion in media with high pH.)

Iron does not undergo corrosion at pH greater than 10. This is due to the formation of protective coating of hydrous oxides of iron. Between pH 10 and 3, the presence of oxygen is essential for corrosion. If the pH is less than 3, corrosion occurs even in the absence of oxygen.

8. Temperature: Higher the temperature, higher is the rate of corrosion.

In general, the rate of a chemical reaction increases with increase in temperature. Corrosion is one such chemical reaction. Therefore, the rate of corrosion increases as the temperature increases.

Increase in temperature increases the ionic conductivity of the corrosive medium. This also contributes to the increase in corrosion rate.

Corrosion control

1. Anodizing (Anodizing of aluminum): When aluminum metal is made anodic in an electrolytic bath with sulphuric acid or chromic acid as the electrolyte, a thin layer of aluminium oxide (Al2O3) is formed on the surface. This process is called anodizing of aluminium or anodic oxidation of aluminum.

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When anodized aluminium is exposed to a corrosive environment. the Al2O3 layer on the surface acts as a protective coating. Hence corrosion is prevented.

Other metals such as Mg, Ti etc. can also be anodized.

(Note: On anodizing, Al2O3 is formed on the surface as a porous layer. The layer may be made compact by sealing, which involves heating with boiling water or steam. During sealing, Al2O3 is converted into Al2O3.H2O which occupies higher volume. Therefore, the pores are sealed.)

Applications: Metals such as Al, Mg, Ti etc. are anodized to control corrosion.

2. Phosphating: Converting the surface metal atoms into their phosphates by chemical or electrochemical reactions is called phosphating.

The phosphating bath contains three essential components: (i) free phosphoric acid, (ii) a metal phosphate such as Fe, Mn phosphate and (iii) an accelerator such as H2O2. Phosphating not only improves the corrosion resistance but also imparts good paint adhesion quality to the surface.

Applications: Phosphate coating is given as an under layer before painting the car bodies, refrigerators and washing machines.

3. Organic coating: (Coating with paints and enamels): Metal corrosion may be controlled by the application of an organic coating of paint on the surface. The organic coating acts as a barrier between the metal surface and the corrosive environment.

A primer coat such as phosphate coating is generally applied as an undercoat before painting.

Organic coatings are applied by different methods such as brushing, spraying, roller coating etc.

4. Galvanizing : Galvanizing is the process of coating a metal surface such as iron with zinc metal. Galvanizing of iron is an example of anodic metal coating on the surface of a cathodic metal.

Galvanization is carried out by hot dipping method. It involves the following steps.

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1. The metal surface is

washed with organic solvents to remove organic matter on the surface.2. Rust is removed by washing with dilute sulphuric acid.3. Finally, the article is washed with water and air-dried.4. The article is then dipped in a bath of molten zinc. (Molten zinc is covered with a flux

of ammonium chloride to prevent the oxidation of molten zinc.)5. The excess zinc on the surface is removed by passing through a pair of hot rollers.

Application: Galvanization of iron is carried out to produce roofing sheets, fencing wire, buckets, bolts, nuts, pipes etc.

(Note: Even if the Zn coating falls off at some places, the base metal (Fe) does not get corroded at those places. This is because the base metal acts as cathode. In corrosion process, the cathodic metal always remains unaffected.)

(Note: Galvanized articles are not used for preparing and storing food because zinc dissolves in dilute acids producing toxic zinc compounds)

5. Tinning: Tinning is the process of coating the surface of a base metal (such as iron) with tin. Tinning of iron metal is an example of cathodic metal coating on an anodic base metal.

Tinning of iron is carried out by hot dipping method. It involves the following steps.

1. The metal surface is washed with organic solvents to remove organic matter on the surface.

2. Rust is removed by washing with dilute sulphuric acid.3. Finally, the article is washed with water and air-dried.4. It is then passed through molten zinc chloride flux. The flux helps the molten tin to

adhere strongly on the surface.5. It is then dipped in a bath of molten tin.

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Dc Power

Al2O

3H

2CrO

4

CathodeAnode A

6. The excess tin on the surface is removed by passing through a series of rollers immersed in palm oil. The oil prevents the oxidation of tin coating.

Applications: Tin-coated steel is used for manufacturing containers.

(Note: In tin-coated iron, if a small crack is formed in the tin coating, an intense and accelerated corrosion of iron occurs at the exposed region because of the formation of a small anodic area (iron) and large cathodic area (tin)).

(Note: Copper utensils are coated with tin to prevent contamination of food with poisonous copper salts.)

6. Corrosion inhibitors:

Corrosion inhibitors are chemical substances which are added in small quantities to the corrosive environment to decrease the rate of corrosion. Inhibitors slow down the anodic reaction or the cathodic reaction generally by forming a protective film on the anodic region or the cathodic region.

There are two types of corrosion inhibiters namely anodic corrosion inhibiters and cathodic corrosion inhibiters.

Anodic corrosion inhibitors: Compounds such as chromates, molybdates and tungstates are used as anodic corrosion inhibitors. They react with the surface atoms at the anodic region and form a protective film of oxide or metallate. This protective film acts as a barrier between the metal surface and the corrosive environment.

Anodic corrosion inhibitors must be added in sufficient quantities. If insufficient quantity is added, then a part of the anodic region is covered with the protective film leaving the remaining anodic region exposed to the environment. Formation of small anodic area results in intense corrosion.

Cathodic corrosion inhibitors:

The cathodic reaction may be (i) liberation of hydrogen or (ii) absorption of oxygen.

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(i) When the cathodic reaction is liberation of hydrogen, the cathodic reaction can be retarded (a) by forming an adsorbed layer on the cathode surface or (b) by increasing the hydrogen over voltage.

(a) When compounds such as amines, urea and thiourea are added to the corrosion medium, they are adsorbed on the cathodic region forming a protective film. The protective film prevents the migration of H+ ions on to the cathodic surface.

(b) When oxides of arsenic and antimony are added to the corrosive medium, they form a metallic film on the cathode surface. Since the hydrogen over voltage on these metals is high, hydrogen liberation is retarded.

(ii) When the cathodic reaction is absorption of oxygen, salts such as ZnSO4, MgSO4 etc. are added to the corrosive medium. The cations of these salts (Zn2+, Mg2+) react with the hydroxyl ions formed at the cathode, depositing insoluble hydroxides (Zn(OH)2, Mg(OH)2 ) on the cathode surface. The hydroxide film acts as a protective film. It prevents the migration of oxygen to the cathode surface.

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